Method of detecting flaws in glass surfaces and solution used therefor



United States Patent 3,171,768 METHGD 0F DETECTING FEJAWS IN GLASS SUR- FACES AND SOLUTIGN USED THEREFOR William C. Leveragood, Muncie, Ind, assignor to Bail Brothers Company, Incorporated, Muucie, End, a corporafion of ludiana No Drawing. Filed Apr. 25, 1961, Ser. No. 113,584 13 Claims. (Cl. res-2a This invention relates to a method of and composition for detecting minute flaws and discontinuities on surfaces of glass articles, and more particularly to a method of visually developing existing surface flaws, such as submicroscopic Griflith cracks, to determine their effects on the glass articles.

The existence of minute flaws or crack patterns was first postulated with the theoretical studies of A. A. Griffith, in his paper The Phenomenon of Rupture and Flow in Solids, published in 1920 in the Philosophical Transactions of the Royal Society of London, volume A221, pages 163 to 198. It was Grifliths theory that minute sub-microscopic flaws in the surface of solids was the reason why the actual strength of the solids did not correspond to their theoretical strength.

I have confirmed these theories, particularly in connection with mold-blown glass articles, and have discovered that these minute surface flaws do indicate a reduction in the strength of the glass article below its theoretical strength. In addition to this, observation of the location of the flaws on the article, their grouping and spacing with relation to each other, together with the line of direction along the surface of the article of the group or groups of cracks, give an indication of the defeet in the apparatus, including the mold employed, in making the particular article involved. These surface flaws also give some indication of the procedural defect or defects which may be involved in the production of the article.

Subsequent to the Griffith report, E. N. Andrade and L. C. Tsien published a paper entitled On Surface Cracks in Glass in the Proceedings of the Royal Society of London, volume A159, pages 346 to 355, dated March 2, 1937. Andrade and Tsien showed that these minute flaws, which they chose to call Griflith cracks, developed and became visible on a glass surface when the surface was heated in an evacuated chamber containing a vapor of pure sodium. A sample or" glass to be tested was suspended within a Pyrex glass tube, and a particle of sodium was placed at the bottom of the tube. The whole tube Was then heated in an electric furnace. A high vacuum was maintained in the tube, and an average temperature of about 350 C. was maintained in the region of the test sample located in the tube. The specimen was exposed for-about /2 hour to the sodium vapor produced in the heated tube. Andrade and Tsien used hydrofluoric acid as well as sodium vapor in-an attempt to develop the Grifiith cracks. However, they found'that the hydrofluoric acid only revealed superficial marks such as polishing scratches and drawing marks, and did not visually develop minute sub-microscopic cracks, as did the sodium vapor.

The sodium vapor method for developing such minute surface flaws and cracks requiredthe use of speciallyprepared samples and rather complex laboratory apparatus. The size of the sample to be tested was extremely limited since it had to be enclosed Within a Pyrex glass tube. Also, the procedure of setting up the apparatus, evacuating the tube, heating up the furnace to vaporize the sodium, and exposing the specimen to the desired temperature was time-consuming and requiredfrom 4 to 6 hours to test a single sample.

An object of my invention is to provide a rapid method of visually developing existing minute flaws and discontinuities, such as Grifiith cracks, on glass surfaces, without the use of an involved special procedure and complex laboratory apparatus;

Another object of my invention is to provide a method for the detection of mechanically, thermally and chemically produced surface cracks in glass;

A further object of my invention is to provide means for investigating and determining the effect of surface flaws on the'strength of glass;

An additional object of my invention is to provide a diagnostic tool for determining stress conditions in the surface of glass;

Another object of my invention is to provide means for studying the effect of foreign substances in contact with glass surfaces;

Another object of my invention is to provide a process for determining the effect on glass surfaces of aging and weathering;

A still further object of my invention is to provide a new solution for visibly developing existing minute flaws and discontinuities, such as sub-microscopic cracks, in glass surfaces. 7

The foregoing and other objects, features and advantages of my invention are more fully described in the following specification.

As pointed out in my co-pending application Serial No. 724,127, filed March 26, 1958, and entitled Method for Detecting and Determining the Effect of Minute Flaws on Glass Surfaces, now abandoned, of which this application is a continuation-in-part, I have discovered that these minute sub-microscopic flaws or Grifiith cracks may be developed to the extent of being made visible tothe naked eye by using a special etching technique. My new method appears to be based upon the slow action of dissociated sodium fluoride on the glass surface. To obtain this action, I prefer toutilize an aqueous solution of sodium fluoride and hydrochloric acid. An excess of sodium fluoride to hydrochloric acid is necessary for operable results. A portion of this excess NaF settles to the bottom of the solution, leaving a clear liquor as the etching reagent. The proportions of the ingredients constituting my improved etching composition must be maintained within restricted limits in order "to produce operative results. I have found that the ratios of sodium fluoride and water to hydrochloric acid must be maintained Within about 2 to 4 parts by weight NaF and between about 25 to 55 parts by weight H O for each part by weght HCl in order to achieve the desired etching results.

My nowprcferred etching solution, which :1 have found produces optimum results, comprises 2 parts by weight of a 10% solution of sodium fluoride mixed with one part by weight of 10% hydrochloric acid at about 20 C. Through experimentation I have found that operative re sults are also produced when 2 parts by weight of a 10% solution of sodium fluoride are mixed with 1 part by weight of 5% hydrochloric acid, and then Z'parts by weight of a 15% solution of sodium fluoride are mixed with 1 part by weight of 10% hydrochloric acid. However, when the concentration of the hydrochloric acid in my preferred solution is increased above 10% or decreased below 5%, "no flaw patterns are developed by the reagent. Similarly, when the concentration of the sodium fluoride is reduced below 10%, no flaw patterns are developed. When, on the other hand, the concentration of sodium fluoride is increased above 15%, a film of etch crystals is deposited upon the surface of the treated glass and thus deleteriously 'aifects the visual development of the desired flaw patterns. Further experimentation has shown that the desired reaction is not obtained when other fluoride salts or other mineral acids are substituted for the sodium fluoride and hydrochloric acid, respectively.

I have experimented with straight solutions of NaF in various concentrations. However, these solutions merely etch the surface of the glass but do not bring out or visually develop existing minute flaws, such as Grifi'ith cracks and fracture patterns. Similar unsatisfactory results merely producing an extended etch were obtained by using solutions containing only HF. Andrade and Tsien in their work with sodium vapor, also reported that I-IF alone will not develop these patterns of Grifiith cracks. I have also tried combining molar solutions of NaCl and HF and found that such a mixture'did not satisfactorily develop the crack patterns. However, when sufiicient is added to NaCl and HF mixtures, the resulting solution produces the desired etching characteristics by visually developing the sub-microscopic surface flaws. Accordingly, I have found that a satisfactory etching solution can be produced by utilizing about 3 parts by Weight NaCl, about 1.5 to 5 parts by weight NaF, and between about 45 to 100 parts by weight water for each part by weight HF.

When the inside surface of a container is to be examined, the solution is simply poured therein and allowed to etch the internal surface for a pre-determined time. When it is desired to examine the outside surface of a container, it is placed in a suitably lined tank and the 'containeris weighted down by'filling it with water or some suitable material. The etching solution is then poured into the tankaround the outside of the container and allowed to etch the exterior surface. When it is desired to examine small fragments, for example, samples of plate glass, the samples are merely immersed in a tank containing the preferred etching solution and are generally held in an upright or vertical position during the etching period. I have found that the optimum etching time with the preferred solution is about minutes. However, fractures and defects may be seen in some cases after only a 5 minute etch, and may also be observed after 2 hours of etching. The solution is used preferably at room temperature, and temperature does not appear to be critical.

Fresh or pristine glass such as a freshly fractured surface may be placed directly in my novel etching solution and the fracture patterns will develop. If the sample has aged, it is desirable to first clean the glass and remove any weathered film, grease, or dirt from the surface before etching. Cleaning may be accomplished by rubbing the surface to be examined with a cotton swab saturated with a mud-like precipitate of calcium carbonate. Following the cleaning, the sample is rinsed with water, dried, and then etched in the same manner as fresh glass.

My preferred etching solution chemically develops existing sub-microscopic surface cracks and flaws so that they become visible. The cracks may be easily observed simply by holding the treated sample before the eyes with a distant light source in the background. However, visual contrast may be increased by rubbing a fine powder, such as graphite, into the cracks and then viewing the sample as noted. Also, when examining the interior of a container, I have found that by dusting the inside of the container with a fine aluminum powder after utilizing the graphite treatment, the visibility of the cracks is greatly enhanced.

The following detailed examples of how the invention may be practiced are given merely for purposes of illustration. Obviously, the procedure, apparatus and materials in those examples may be widely varied.

Example 1 The inside surface of a pristine glass bottle is t o be examined. An etching solution is made up of two parts 10% solution of sodium fluoride mixed at room temperature C.) with one part 10% hydrochloric acid. The

mixture is poured into the upright glass bottle and allowed to etch the interior surface thereof for a period of about 15 minutes. At the end of this period, the etching solution is removed and the interior etched surface is dried. Fine graphite powder is then placed in the container and rubbed into the etched cracks. Excess graphite powder is removed from the container, and line aluminum powder is placed therein. The aluminum powder is then'spread about the interior surface of the bottom by rotating the bottle. The bottle may then be easily examined with the naked eye, because the etched cracks stand out as black contrasting lines against a silver background.

Example 2 The exterior surface of an aged glass bottle is to be examined. The surface to be treated is first rubbed with a cotton swab saturated with a mud of precipitated calcium carbonate to produce a clean surface. Following the cleaning, the surface is rinsed with distilled water and then dried by rubbing with a soft absorbent material. The cleaned glass bottle is filled with water, to weight it down, and then placed in an empty tank made of polyethylene, which is larger than the bottle to be examined. My preferred etching solution made up of two parts of 10% solution of sodium fluoride mixed at room temperature (20 C.) with one part 10% hydrochloric acid, is then poured into the tank around the bottle; After fifteen minutes of etching, the bottle is removed from the tank, drained and dried. The etched cracks on the glass surface are then examined by simply holding the treated bottle between the naked eye and a distant light source.

Crystals deposit less readily over the cracks than they do over the surrounding surface. I have found that the crack regions etch more rapidly than the surrounding surface. The reason for the more rapid etch along the cracks may be explained by the fact that there is greater free surface energy around these minute surface discontinuities. It is easier to remove an atom or molecule from an edge than from a surface area.

I have also found that the crack patterns vary with stresses in the glass. By experimentation, it was found that as the stress in a sheet of glass increased, the square root of the crack length proportionately increased.

My examination of crack patterns visually developed with my new process, indicate that the crack patterns are generally more pronounced on the inside surface of a glass container than on the outside surface. In my opinion, the reason for the difference between the inside and outside surface of a glass article, is the manner in which the stresses are created during the article-forming process.- The outside of a newly formed glass container cools quickly when exposed to the atmosphere and sets up or becomes rigid, while the inside of the container which is not as readily exposed to atmospheric temperature, remains somewhat plastic. As the cooling of the container continues, the inside surface cools more slowly than the outside surface and is placed in tension by the colder and more rigid exterior surface. The sub-microscopic flaws or Grifiith cracks have a tendency to form on the inside of the glass container because of these tension stresses. This situation is aggravated in smallmouth containers where the internal heat cannot escape as rapidly as from a wide-mouth container. Consequently, in such instances the temperature difference between the inside andoutside surfaces is greater, and higher stresses develop, causing a greater number of cracks. By the use of my new etching technique and the resulting thermal stress cracks visually developed thereby, I can quickly determine, in a matter of minutes, whether or not the glassware is being properly cooled during production.

Glass fibers are known to have very little residual stress since they cool evenly because of their thinness, and it would therefore be expected that the number of surface cracks in small diameter fibers would be negligible. To check this theory by the use of my new etching technique, fibers drawn from flint glass about 25 centimeters long and about .75 millimeter in diameter, were etched with my new solution. An examination of these etched fibers revealed an absence of crack patterns. Also, a soft glass rod of about 4 millimeters in diameter was similarly etched and examined and found to have only a very few cracks. The absence of these Griffith cracks in the fibers and the fact that only a few are found on small rods, may at least partially explain the high strength of glass fibers and small diameter rods.

Up to this time, I have dealt solely with the submicroscopic flaws or discontinuities in glass surfaces which form in the glass surface due to thermal stress and are not due to mechanical abrasion. Minute fractures occasioned by mechanical injuries to a glass surface do not etch in the same manner as the thermal stress patterns. In the case of scratches, drawing marks, and other superficial markings, my novel method discloses that the etched crystals gather on the cracks or markings as well as the surrounding region.

Crack patterns do not apepar to cross one another, but rather meet at angles of roughly 90. This phenomenon can probably best be explained by theorizing the formation of such cracks. When a tension stress is developed in a glass surface, crack patterns are formed which propagate in a direction normal to the principal stress. After they form, the tension stress is decreased laterally and the principal stress is then formed parallel to the crack direction. If a second crack approaches one that is already formed, the approaching crack will tend to align itself at right angles with the principal stress, thus giving an angle of intersection with the formed crack of approximately 90.

These flaws undoubtedly form very rapidly, and perhaps even in a matter of a few microseconds. When the cooling stresses exceed the elastic strength of the glass, crack patterns are produced which remain frozen in the surface. My etching method therefore provides a diagnostic tool for examination of existing stress conditions when the glass article is formed. In fact, I have observed that the cracks form in a manner that is indicative of the stress pattern within the glass surface. A stress pattern was found to be repeating in the same general lo-' cation in jars or samples taken from the same mold at about the same time. This finding indicates that the stress conditions are very similar, if not identical, in samples taken from the same mold, under similar conditions. My method provides a rapid, non-destructive means of study stress conditions formed in a glass article during its manufacture so that various changes in the forming procedure may be made in the effort to produce glassware of superior quality.

I have also found that glass cutting wheels produce localized crack patterns which may be detected with my etching technique. These crack patterns are concentrated along the cut and are probably formed when the sample is stressed by the cut-ting wheel. Thus, it may be seen that my method may also be used in the study of glass cutting processes.

Also, with the use of my new etching method, I have found that there is a direct relationship between the number of fractures on a glass surface and the strength of the glass. Six samples were taken from each of a number of container forming machines, and the impact strength was determined for each sample. After breaking each sample, during the determination of its impact strength, a fragment of the broken sample taken near the failure zone, was etched with my new solution, and the number of cracks on the inside surface of the container were counted per unit length. In comparing glass articles it was found that those having the few number of cracks or fractures per unit of length, were the stronger. This finding confirms Grifiiths postulated theory that minute sub-microscopic flaws in the surface of solids, such face, as distinguished from 6 as glass, weaken the solid so that it does not possess its theoretical strength.

The effect of these fractures on the impact strength was also brought out graphically in my recent study on glass gallon containers. Containers having an average impact breaking velocity of about inches per second disclosed very few crack patterns. Other containers of the same size, shape and design having an average imp act velocity of only 60 inches per second were found to have pronounced fractures. It was interesting to note that the low impact containers also contained partially rescaled cracks. With the use of my new etching technique, I observed that the stress cracks extended through the partially rescaled or healed cracks, thus indicating that the heavy, healed-over cracks offered no stress barrier for the later formed thermal cracks.

The effects of coating or lubrication glass forming molds on glass surfaces is also disclosed by my etching technique. The patterns produced on the glass surfaces by the mold, doping compound, lubricant, etc. a're distinctly different from the stress crack patterns. The difference between the crack patterns and the molding marks may be readily detected by direct visual observation after etching the surface with my new solution.

The efieot of aging a glass surface may also be studied with this technique. A series of containers taken at the same time from the same mold were aged in air for various lengths of time and etched Without any recleaning. I observed that the number of fractures appeared to decrease with the length of aging time. Also localized attack or effects from the atmosphere could be observed on the surfaces of the container.

The etched pattern formed by my new etching solution is very sensitive to any chemical alterations of the surface, and therefore, I have found it to be useful as a means of studying the effect of various foods, drugs, chemicals, etc. on the glass surface.

As many apparently widely different embodiments of this invention may be made without departing'from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments hereof except asdefined in the appended claims.

What I claim is:

1. A method of bisually-developing existing minute flaws in the nature of cracks, defects, etc., on a glass surrnerely etching the glass surface, to make the flaws readily visible to the naked eye which comprises, preparing an aqueous treating solution of sodium fluoride and hydrochloric acid in which the sodium fluoride is in excess of the hydrochloric acid, and applying the solution to the glass surface so proportioned in its relative quantities of water, sodium, fluoride and hydrochloric acid as to effect a preferential-etching action on and visual development of the flaws with respect to the remainder of the glass surface.

2. A method as defined in claim 1 wherein the aqueous solution is prepared in the for-m of a clear liquid and an excess proportioning of the sodium fluoride to the hydrochloric acid is maintained during the visual development of the flaws.

3. A method as defined in claim 1 wherein, the treating solution is then washed off the glass surface, and the surface is rubbed with graphite in fine powder form to increase the visual contrast of the visually-developed flaws.

4. A method as defined in claim 2 wherein a fine aluminum powder is dusted-on the glass surface after the rubhing-on of the graphite to further enhance the visual contrast of the flaws.

5. A method of visually-developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished from merely etching the glass surface, to make the flaws readily visible to the naked eye which comprises, subjecting the glass surface to a treating solution consisting essentially of about 2 to 4 par-ts by weight of sodium fluoride and about 25 '7 to 55, parts by weight of water with each part by weight of hydrochloric acid, and maintaining the glass surface in contact with the solution while visually developing the flaws in a preferential manner with respect to the remaindium fluoride mixed with about 1 part by weight of about a 5 to aqueous solution of hydrochloric acid, while maintaining the sodium fluoride in excess of the hydrochloric acid; and maintaining the glass surface in contact with the solution until the flaws are preferentially-visually developed with respect to remaining portions of the surface.

7. A method as described in claim 6 wherein. the treating solution is maintained in contact with the glass surface for at least 5 minutes.

8. A method as defined in claim 6 wherein the glass surface is first cleaned to remove film, grease, dirt, etc., by rubbing it with a mud-like precipitate of calcium carbonate, and is then rinsed and dried before the application of the treating solution.

9. A method of visually-developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished from merely etching the glass surface, to make the flaws readily visible to the naked eye which comprises, subjecting the glass surface to a treating solution consisting essentially of about 2 parts by weight of about a 10% aqueous'solution of sodium fluoride mixed with about 1 part by weight of about a 5% aqueous solution of hydrochloride acid. v

10. A method of visually-developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished from merely etching the glass sur face, to make the flaws readily visible to the naked eye which comprises, subjecting the glass surface to a treating solution consisting essentially of about 2 parts by i and about 1 part by Weight of hydrochloric acid.

8 Weight of about a 15% aqueous solution of sodium chloride mixed with about 1 part by weight of about a 10% aqueous solution of hydrochloric acid.

11. A method of visually-developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished'from merely etching the glass surface, to make the flaws readily visible to the naked eye which comprises, subjecting the glass surface to a treating solution consisting essentially of about 3 parts by weight of sodium chloride, about 1.5 to 5 parts by weight of sodium fluoride, and about to 100 parts by weight of water for each part by Weight of the reaction-product hydrogen fluoride.

12. A flaw developing solution for use in a method of visually developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished from merely etching the glass surface, to make the flaws readily visible to the'naked eye which consists essentially of about 2 to 4 parts by weight of sodium fluoride, about 23 to parts by Weight of water,

13. A flaw developing solution for use in a method of visually developing existing minute flaws in the nature of cracks, defects, etc., on a glass surface, as distinguished from merely etching the glass surface, to make the flaws readily visible to the naked eye which consist essentially of about 2 parts by weight of about a 10 to 15% aqueous solution of sodium fluoride mixed with about 1 part by weight of about a 5 to 10% aqueous solution of hydrochloric acid.

References Cited by the Examiner UNITED STATES PATENTS Re. 17,682 5/30 Smith 15625 XR 2,622,016 12./52 Gilstrap et al 15625 XR 3,130,098 4/64 Levengood 156-24 FOREIGN PATENTS 273,046 6/27 Great Britain.

EARL M. BERGERT, Primary Examiner. CARL F. KRAFFT, Examiner. 

1. A METHOD OF VISUALLY-DEVELOPING EXISTING MINUTE FLAWS IN THE NATURE OF CRACKS, DEFECTS, ETC., ON A GLASS SURFACE, AS DISTINGUISHED FROM MERELY ETCHING THE GLASS SURFACE, TO MAKE THE FLAWS READILY VISIBLE TO THE NAKED EYE WHICH COMPRISES, PREPARING AN AQUEOUS TREATING SOLUTION OF SODIUM FLUORIDE AND HYDROCHLORIC ACID IN WHICH THE SODIUM FLUORIDE IS IN EXCESS OF THE HYDROCHLORIC ACID, AND APPLYING THE SOLUTION TO THE GLASS SURFACE SO PROPORTIONED IN ITS RELATIVE QUANTITIES OF WATER, SODIUM, FLUORIDE AND HYDROCHLORIC ACID AS TO EFFECT A PREFERENTIAL-ETCHING ACTION ON AND VISUAL DEVELOPMENT OF THE FLAWS WITH RESPECT TO THE REMAINDER OF THE GLASS SURFACE.
 12. A FLAW DEVELOPING SOLUTION FOR USE IN A METHOD OF VISUALLY DEVELOPING EXISTING MINUTE FLAWS IN THE NATURE OF CRACKS, DEFECTS, ETC., ON A GLASS SURFACE, AS DISTINGUISHED FROM MERELY ETCHING THE GLASS SURFACE, TO MAKE THE FLAWS READILY VISIBLE TO THE NAKED EYE WHICH CONSISTS ESSENTIALLY OF ABOUT 2 TO 4 PARTS BY WEIGHT OF SODIUM FLUORIDE, ABOUT 23 TO 55 PARTS OF WEIGHT OF WATER, AND ABOUT 1 PART BY WEIGHT OF HYDROCHLORIC ACID. 